two ways. One way is via reuptake transporter proteins located in
the cell membrane of the axon terminal. When neurotransmitter
bangs into its corresponding reuptake transporter protein, it will
bind to the transporter and be moved from the exterior of the cell
back into the interior, and so out of the synaptic cleft. Most known
neurotransmitters are removed from the synaptic cleft by specific
reuptake transporter proteins. A second mechanism of inactivation
involves an enzyme in the synaptic cleft; this mechanism is used
where acetylcholine is the neurotransmitter. An enzyme called acetyl-
cholinesterase rapidly breaks the acetylcholine molecule into two
pieces, thereby inactivating it as a functional neurotransmitter.
These processes of inactivation—either reuptake or enzymatic—
act very rapidly and efficiently to remove neurotransmitters from the
synapse. It is essential that this process be rapid and efficient, because
the synapse must be ready to receive another nerve signal, should it
occur, and be able to discriminate a new signal from the remnants of
a previous signal. It is like having written a bunch of stuff on a black-
board and then needing to erase the board in order to make room to
write new stuff. Otherwise, the board would become so cluttered that
it would be hard to tell when new information appears.
All this takes place very rapidly, in perhaps 1 millisecond or so.
The concept of chemical neurotransmission was first demonstrated
by Otto Loewi (1873-1961) in 1920. And the idea for the experiment
demonstrating chemical neurotransmission occurred to him ina
dream. Loewi had been investigating neural regulation of the heart
in his laboratory when he awoke one night in the spring of 1920 and
jotted down some ideas for an experiment on a slip of paper. The next
morning he found that he couldn’t decipher his scribbles from the
night before. However, feeling that some good ideas had occurred to